Red and infrared light-emitting diodes (LEDs) are utilized widely, such as in communication, a light source for various sensors, nightlights etc.
For example, since light having a peak wavelength of 660-720 nm is a red light source that a person can recognize, it is widely used as a light source of an outdoor display; a safely-affiliated sensor which uses light at a wavelength range having a high output and therefore it is desirable that the light can be recognized by visual inspection; or a bar code reader and a light source of a medical application such as an oxymeter. In addition, since light having a peak wavelength of 760-850 nm is in the wavelength range that output is high, it is the most suitable wavelength range for a light source of various sensors or a surveillance camera, or infrared lighting for a video camera. Particularly, since the AlGaAs active layer in the wavelength range has a high-speed response, it is suitable to be used in optical communication or a high-performance photo coupler. On the other hand, it begins to be utilized as light sources in medical fields such as in a venous authentication system because of the characteristic of the wavelength.
In addition, plant growth using an artificial light source has been studied. In particular, attention is being paid to a cultivation method illuminating by a light-emitting diode which has excellent monochromaticity and in which energy saving, long lifetime, and a reduction in size are possible. With respect to photosynthesis, light having a wavelength of around 660 nm to 670 nm is the most preferable light source having high reaction efficiency. Moreover, the conventional studies have confirmed that the infrared light having a peak wavelength of 730 nm, as one of the wavelengths which are preferable for shape control of plant upbringing, is effective.
It is preferable that LEDs have a high output in order to improve performance of each equipment for the above uses.
For example, as a conventional infrared light-emitting diode, the light-emitting layer including AlGaAs obtained by using a liquid phase epitaxial growth method is put to practical use, and various investigations for realizing high output have been done. For example, an infrared light-emitting diode was produced by growing an epitaxial AlGaAs layer thickly on a GaAs substrate by using a liquid phase epitaxial growth method, wherein the layer is transparent at the emitting wavelength, and removing the GaAs substrate to use the AlGaAs layer as a substrate. Recently, the structure of the infrared light-emitting diode (a so-called transparent-substrate type) shows the highest output (For example, Patent Documents 1-5).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-30150    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-274454    Patent Document 3: Japanese Unexamined Patent Application Publication No. 6-268250    Patent Document 4: Japanese Unexamined Patent Application Publication No. 6-13650    Patent Document 5: Japanese Unexamined Patent Application Publication No. 8-293622